1. Field of the Invention
The present invention relates to a semiconductor optical amplifier for amplifying optical signals, and more particularly to a gain-clamped semiconductor optical amplifier capable of providing constant gain for output optical signals.
2. Description of the Related Art
When reaching a gain saturation region, a semiconductor optical amplifier (hereinafter, referred to as an SOA) cannot perform its function as an amplifier because of an interference phenomenon between channels. In order to prevent such a gain saturation phenomenon, conventional SOAs have used methods that stimulate lasing in the amplifiers to clamp the gain of the amplifiers.
Lasing methods, which are used to clamp the gain of the SOAs, may be largely classified into a Distributed Feedback (hereinafter, referred to as a DFB) method and a Distributed Bragg Reflector (hereinafter, referred to as a DBR) method.
FIG. 1 is a side cross-sectional view schematically showing the construction of a conventional DFB SOA 100. The SOA 100 includes a substrate 110, a grating layer 120, a gain waveguide 130, and a clad 140.
The grating layer 120 is laminated on the substrate 110 and includes a grating 125 having a predetermined period with respect to the total length of the grating layer 120. The gain waveguide 130 is laminated on the grating layer 120 and amplifies an optical signal input to the gain waveguide 130. The clad 140 is laminated on the gain waveguide 130. Since the clad 140 and the grating layer 120 have refractive indices smaller than the refractive index of the gain waveguide 130, the optical signal is confined in the gain waveguide 130.
However, in the DFB SOA 100, since the grating layer 120 is formed under the gain waveguide 130 having electron density and photon density which vary according to the input of electric current and an optical signal, the effective grating period of the grating 125 varies according to such exterior factors. The variation of the effective grating period of the grating 125 causes instability of a lasing due to the grating 125. Therefore, since the gain property of the SOA 100 is unstable, it is not possible to obtain a clamped gain property.
In the conventional DBR method, a passive waveguide is formed. A grating layer including gratings is disposed under the passive waveguide. In the DBR method, the grating layer is disposed under the passive waveguide so that the grating layer does not experience variation of electron density according to input of electric current. This prevents the effective grating period from being easily changing. Therefore, a stable lasing can be obtained and the gain property of the SOA is also stabilized. However, construction and manufacture of the gain waveguide and the passive waveguide are difficult in comparison with the above-mentioned DFB method. In addition, optical coupling loss exists between the gain waveguide and the passive waveguide, so that properties of the SOA are deteriorated. In the DBR SOA, methods for forming the passive waveguide in contact with the gain waveguide include a butt-joint method and a dual waveguide method.
FIG. 2 is a side cross-sectional view schematically showing a construction of a conventional DBR SOA 200 employing the butt-joint method. The SOA 200 includes a substrate 210, a grating layer 220, a gain waveguide 240, first and second passive waveguides 230 and 235, and a clad 250.
The grating layer 220 is laminated on the substrate 210 and includes first and second gratings 222 and 224 formed at a first end portion and a second end portion of the grating layer 220. The gain waveguide 240 is laminated on the grating layer 220 to be out of contact with the first and the second gratings 222 and 224. The gain waveguide 240 amplifies an optical signal input to the gain waveguide 240. The first passive waveguide 230 is laminated on the grating layer 220 to be in contact with one end of the gain waveguide 240, and the second passive waveguide 235 is laminated on the grating layer 220 to be in contact with the other end of the gain waveguide 240. The clad 250 is laminated on the gain waveguide 240, the first passive waveguides 230, and the second passive waveguides 235. The clad 250 and the grating layer 220 have refractive indices smaller than those of the gain waveguide 240 and the passive waveguides 230 and 235.
However, it is difficult to manufacture the DBR SOA 200 employing the butt-joint method. In addition, reflection inevitably occurs at contact portions between the gain waveguide 240 and the first passive waveguides 230, and between the gain waveguide 240 and the second passive waveguides 235. Furthermore, optical coupling efficiency is not perfect. Therefore, properties of the SOA 200 are deteriorated.
FIG. 3 is a side cross-sectional view schematically showing a construction of a conventional DBR SOA 300 employing the dual waveguide method. The SOA 300 includes a substrate 310, a grating layer 320, a passive waveguides 330, a gain waveguide 340, and a clad 350.
The grating layer 320 is laminated on the substrate 310 and includes first and second gratings 322 and 324 formed at both sides of the grating layer 320. The passive waveguides 330 is laminated on the grating layer 320. The gain waveguide 340 has a length shorter than that of the passive waveguides 330 and is laminated on a central portion of the passive waveguides 330. The gain waveguide 340 amplifies an optical signal input to the gain waveguide 340. The clad 350 is laminated on the passive waveguides 330 to surround the gain waveguide 340. The grating layer 320 and the clad 350 have refractive indices smaller than those of the gain waveguide 340 and the passive waveguide 330. An optical signal, which is input to one end of the passive waveguide 330, progresses inside of the passive waveguide 330. The optical signal is then transferred to the gain waveguide 340 to be amplified. Afterward, the optical signal is transferred to the passive waveguide 330 to be output through the other end of the passive waveguide 330.
However, there exists limitations in the optical coupling efficiency between the gain waveguide 340 and the passive waveguide 330, therefore properties of the DBR SOA 300 are also deteriorated.
Accordingly, there exists a need in the art for improved semiconductor optical amplifiers.